The present invention relates generally to diagnostic techniques. More specifically, the present invention relates to the testing of blood units for viral contamination.
In a variety of therapies, such as transfusion and transplants, body fluids, especially blood components, such as red blood cells, platelets, plasma, and bone marrow, are infused from one or more individuals into a patient. Although such therapies provide treatments, some of which are life saving, that cannot otherwise be provided, due to the transmission of infectious diseases there may be potential risks to such therapies.
For example, it is known that blood can carry infectious agents, such as hepatitis virus, human immunodeficiency virus (an etiological agent for AIDS), cytomegalovirus, Epstein Barr virus, and herpes virus. Although screening methods exist to identify blood that may include such viruses, current screening methods do not assure that every blood unit that contains such a virus is identified.
In this regard, one of the difficulties in testing blood components for viral contamination, such as HIV, is that many current diagnostic tests are based on an identification of antibodies. Accordingly, they will only exhibit a positive test result if the blood unit includes antibodies for the virus, e.g., anti-HIV. With respect to intracellular viral infections, an individual, however, does not generate antibodies immediately upon infection. Rather there is a "window period" that extends from the initial infection of the patient with a virus to the generation of antibodies. When an individual is in this window period, diagnostic tests that are based on antibodies will not identify the individual, or the blood unit, as being infected. But, even though antibodies are not present, the blood unit can still transmit an infection.
It is believed that this window period, with respect to HIV infection, extends from approximately six weeks to 48 months. During this time period, an individual who has been infected with HIV and accordingly, whose blood will transmit same, will register a negative antibody response. Therefore, current screening methods will not identify as viral contaminated a blood unit from an individual who is infected with HIV but who has not generated anti-HIV.
In order to identify blood units that may be contaminated because an individual is within the window period, recent attempts have focussed on the use of nucleic acid sequencing diagnostic techniques. Specifically, attempts have been made to use polymerase chain reaction (PCR) techniques for detecting nucleotide sequences for HIV virus.
A number of methods of using PCR are known. Briefly, in one PCR method, a sample containing DNA is placed in a reaction tube including appropriate buffers, nucleoside triphosphates, a thermostable DNA polymerase and oligonucleotide primers complementary to the ends of a region of DNA of interest. Initially, to denature the double-stranded DNA under study, the temperature of the reaction is rapidly increased. The temperature is then decreased allowing the oligonucleotide primers to anneal to their complementary sequences. By increasing the temperature, a DNA extension occurs at the optimal temperature of activity for the polymerase. By repeating these cycles of denaturation-annealing-extension, a single sequence of a few hundred base pairs can be amplified. This amplification can be in the range of a factor of 10.sup.6 and detected with relative ease. Conway, "Detection of HIV-1 by PCR in Clinical Specimens", "Techniques in HIV Research" Stockton Press (1990)
Although nucleic acid sequencing techniques are very sensitive, they are also sample specific. Moreover, the samples of the blood units so tested are rendered unusable for therapeutic applications. Because of this, currently nucleic acid sequencing diagnostic methods do not provide a method for insuring that blood units do not have viral contamination.
For example, if one were to attempt to use a PCR technique to test a sample of a blood unit using standard technology, one could not insure that the unit did not contain viral contamination even if the test was negative. In this regard, because a PCR test is sample specific unless the whole blood unit was tested, it would not accurately reflect that the unit did not include viral contamination; if only a 10 ml sample were tested, the remaining 290 ml of a 300 ml could contain a viral contaminant.
A PCR test will only determine if there is a viral agent in the sample being assayed, not in the entire unit. Because the test destroys the viability of the component, heretofore, it was not possible to test the entire component to ensure that it is not contaminated with, for example, HIV.
Accordingly, there is a need for improved diagnostic testing of blood units to determine viral contamination during the "window period" of contamination.